A wheel type hydraulic shovel, for example, as shown in FIG. 10 has been known as a hydraulic construction machine equipped with the above type of hydraulic control apparatus. In the figure, a reference numeral 4 represents a hydraulic motor for vehicle travelling, and rear wheels 103 are driven through a transmission 101 and a propeller shaft 102 by the rotation of the hydraulic motor 4 to allow a vehicle to travel. A boom 104 serving as a part of a front attachment is upwardly and downwardly moved through a lengthening and shortening operation of a boom cylinder 21. In this specification, a digging work and the like using the front attachment while the vehicle does not travel is referred to merely as "work", and these works are distinguished from a term "travel".
As one of the hydraulic construction machines as described above has been proposed a machine in which a load sensing control and an input torque limiting control are selectively carried out (for example, as disclosed in Japanese Laid-open Patent Application No. 2-118203). These control systems will be described hereunder with reference to FIG. 11.
FIG. 11 shows a hydraulic circuit for the travel and the work of the hydraulic shovel as described above, and a reference numeral 1 represents a variable displacement hydraulic pump which is driven by a diesel engine 27. The rotational speed of the engine 27 is controlled through rotational member of a governor lever 27b of a governor 27a which is driven, in accordance with an manipulation amount of a fuel lever (not shown) or an operation amount of an acceleration pedal 6a for the travel (hereinafter referred to as a travelling pedal), by a pulse motor 28. A variable displacement hydraulic pump 1 discharges an oil whose flow amount corresponds to the engine rotational speed and the displacement thereof. The discharged oil is communicated through a travel-control valve 2 to the hydraulic motor 4, and further communicated through a work-control valve 20 to a boom-driving hydraulic cylinder 21.
The load sensing control is characterized in that the displacement (hereinafter also referred to as "tilt angle") of the variable displacement hydraulic pump 1 is so controlled that the difference pressure between front and rear stages of the travel-control valve 2 or the work-control valve 20, that is, the difference pressure between a pressure (pump pressure) at an inlet port of the control valve 2, 20 and a pressure (load sensing pressure) at an outlet port thereof is fixed to a constant value to thereby keeping the pump pressure to a value higher than the load sensing pressure by a predetermined target value. The load sensing pressure is a pressure selected by a shuttle valve 29 and higher one of load pressures of the hydraulic motor 4 and the hydraulic cylinder 21.
The system for carrying out the load sensing control as shown in FIG. 11 is equipped with a load sensing regulator 11 whose switching operation is carried out in accordance with the difference pressure between the pump pressure and the load sensing pressure. When the difference pressure between the pump pressure and the load sensing pressure exceeds a pressure which is set by a spring 11a, the load sensing regulator 11 is switched to a position b in accordance with the pressure. At the position b, the pump pressure is communicated to a servo cylinder 12, and the displacement of the hydraulic pump 1 is reduced through a lengthening operation of the servo cylinder 12, so that the delivery flow rate of the pump is reduced. Conversely, when the difference pressure is dropped down to a value lower than the predetermined pressure, the load sensing regulator 11 is switched to a position a, and the servo cylinder 12 is intercommunicated to a tank. Consequently, the displacement as described above is increased, and thus the pump delivery flow rate is also increased. In the system adopting the load sensing control, through the above operation, the tilt angle of the pump is so controlled that the pump delivery flow rate is equal to a required flow rate of the control valve 2 or 20, so that a surplus flow rate of oil can be prevented from being delivered and vain use of the oil due to diaphragm loss can be prevented to improve fuel consumption and operational performance.
Next, the operation of the travelling circuit will be described.
When a forward/reverse travelling switch valve 8 is switched to a forward travelling position (F position) and the pedal 6a of a pilot valve 6 is manipulated, a delivery oil from the hydraulic pump 5 is communicated to a pilot port 2a of a pilot type control valve 2, so that the control valve 2 carries out its switching operation by a stroke amount corresponding to a pilot oil pressure. Through this operation, the delivery oil from the variable displacement hydraulic pump 1 is supplied through a conduit 91, a pressure compensating valve 23, the control valve 2, a conduit 92 and a counterbalance valve 3 to the hydraulic motor 4 to allow the vehicle to run. In addition, the engine rotational speed is controlled in accordance with the operation amount by which the travelling pedal 6a is depressed, so that the travelling speed of the vehicle is dependent on the operation amount of the travelling pedal 6a.
When the travelling pedal is released in the travelling of the vehicle, the connection between the output port and input port of the pilot valve 6 is intercepted, and the output port of the pilot valve 6 is intercommunicated with a tank port which is connected to the tank 10. As a result, the pressured oil which acts on the pilot port 2a is returned through the forward/reverse travelling switch valve 8, a slow return valve 7 and the pilot valve 6 to the tank 10. At this time, the return oil is diaphragmed by a diaphragm 7a of the slow return valve 7, so that the vehicle is gradually decelerated while the pilot-type control valve 2 is gradually switched to a center valve position.
On the other hand, the operation of the working circuit is as follows.
When the work switching valve 20 is switched to a "b" position or "c" position by a manipulating lever 20a, the delivery oil from the hydraulic pump 1 is communicated through the conduit 91, the pressure compensating valve 24, a conduit 94 and the control valve 20 to the boom-driving hydraulic cylinder 21, and the boom 104 as shown in FIG. 10 is upwardly and downwardly moved through the lengthening and shortening operation of the hydraulic cylinder 21. The pressure compensating valves 23 and 24 enable the hydraulic pump 1 to supply the hydraulic motor 4 and the hydraulic cylinder 21 with pressures higher than the load pressures thereof by a predetermined pressure, respectively, so that both actuators can be operated independently of each other.
The hydraulic control apparatus as shown in FIG. 11 is equipped with a torque control servo valve 25 for carrying out the input torque limiting control, and the servo valve 25 is supplied with the delivery pressure of the hydraulic pump 1 as a pilot pressure. When the pilot pressure acting on the servo valve 25 exceeds a pressure set by a limit torque setting spring 25a, the servo valve 25 is switched from the c-position to the d-position as shown in the figure. Through this operation, the delivery pressure of the hydraulic pump 1 acts on the servo cylinder 12, whereby the displacement of the hydraulic pump 1 is reduced, and the torque of the hydraulic pump 1 is kept in a range of the output torque of the engine 27, thereby preventing an overload from being applied to the engine 27. This operation is referred to as the input torque limiting control.
According to the hydraulic control apparatus thus constructed, the displacement of the hydraulic pump is so controlled that it is equal to smaller one of the target displacement by the load sensing control (first target displacement) and the target displacement by the input torque limiting control (second target displacement), and through this operation the improvement in fuel consumption and operating performance and the prevention of overload acting on the engine 27 are accomplished. A reference numeral 26 represents an unload valve which is driven by the difference pressure between the pump pressure and the load sensing pressure, reference numerals 31a and 31b represent a cross-over load relief valve, and CJ represents a center joint.
The hydraulic control apparatus equipped with the load sensing control and the input torque limiting control systems is so constructed that the maximum value of the displacement of the variable displacement hydraulic pump 1 is limited by a value which is determined by the servo valve 25 at the input torque limiting control system side, and in a range below this maximum value the displacement of the variable displacement hydraulic pump 1 is set to the displacement which is determined by the load sensing regulator 11 at the load sensing control system side. As a result of this, the displacement of the variable hydraulic pump 1 is so controlled as to be uniformly set to any one of the input torque limiting control value and the load sensing control value on the basis of the operation amount of the travelling pedal, the operation amount of the manipulating lever or the circuit pressure condition irrespective of variation of an operating circumstance such as "travel" or "work", or an operator's preference.
Therefore, the following two problems occur.